Solar Oven DesignSolar Oven DesignENGR 102ENGR 102Fall 2008Fall 2008

Class NotesClass Notes

General Categories of EnergyGeneral Categories of Energy

KINETIC ENERGYKINETIC ENERGY

Energy in motionEnergy in motionPOTENTIAL ENERGYPOTENTIAL ENERGY

Stored energyStored energy

Forms of EnergyForms of EnergySolar/Light/Radiant Solar/Light/Radiant

EnergyEnergyEnergy from the sunEnergy from the sun

1000 Watts/m1000 Watts/m22 at the at the earth’s surface !!!!!earth’s surface !!!!!

Electrical EnergyElectrical EnergyEnergy as a result of the flow Energy as a result of the flow

of charged particles called of charged particles called electrons or ionselectrons or ions

Forms of EnergyForms of Energy

Mechanical EnergyMechanical EnergyEnergy produced from Energy produced from

mechanical devicesmechanical devices

Chemical EnergyChemical EnergyEnergy that is stored in Energy that is stored in

molecular bonds, the molecular bonds, the forces that hold molecules forces that hold molecules togethertogether

Forms of EnergyForms of Energy

Thermal (Heat) Thermal (Heat) EnergyEnergy

energy in the process of energy in the process of being transferred from one being transferred from one object to another because object to another because of the temperature of the temperature

difference between them.difference between them.

Nuclear EnergyNuclear EnergyEnergy that is trapped inside

each atom

Heat TransferHeat Transfer

Conduction - solidsConduction - solids Convection – gases and liquidsConvection – gases and liquids RadiationRadiation

• Trap heat/solar energy inside a Trap heat/solar energy inside a containercontainer

• Black surfaces adsorb and radiate Black surfaces adsorb and radiate energyenergy

• Shiny surfaces reflect light Shiny surfaces reflect light

solarcooking.org/plans.htm

Solar OvensSolar Ovens

Not just an Academic ExerciseNot just an Academic Exercise• Water/milk pasteurizationWater/milk pasteurization• CookingCooking

Designed by solar engineers to be used Designed by solar engineers to be used in sun rich but fuel poor areas in the in sun rich but fuel poor areas in the world to improve the quality of life and world to improve the quality of life and nutrition of some of the 2.4 billion nutrition of some of the 2.4 billion people who lack adequate cooking fuel people who lack adequate cooking fuel

Solar Oven SocietySolar Oven Society

handout Designhandout Design Flat bottom, flat topFlat bottom, flat top

• Not all sun gets inNot all sun gets in

Alternative:-Aimed OvenAlternative:-Aimed Oven• Incident width = window Incident width = window

width, W= Lwidth, W= L

Solar Oven – Theory Solar Oven – Theory

First law of thermodynamicsFirst law of thermodynamics• Energy in = Energy outEnergy in = Energy out

Joules, BTUs, caloriesJoules, BTUs, calories

• Power out = Power absorbedPower out = Power absorbed Energy/timeEnergy/time Joules/sec, BTU/s, hp, WattsJoules/sec, BTU/s, hp, Watts

• Goal is to determine Power absorbed Goal is to determine Power absorbed and Power out and ultimately to predict and Power out and ultimately to predict Oven Temperature TioOven Temperature Tio

Power Absorbed - factorsPower Absorbed - factors

SunSun• II00 – incident solar power (W/m – incident solar power (W/m22))SS – angle of sun rays with horizon – angle of sun rays with horizon

Size or Area (ASize or Area (Aww))• W – width of glazingW – width of glazing• L – length of glazingL – length of glazing– – angle of window with horizonangle of window with horizon

Material properties of window, ovenMaterial properties of window, oven – – transmissivitytransmissivity• a - absorptivitya - absorptivity

Figure 3-Solar Oven Geometry (handout)

Power AbsorbedPower Absorbed

Sun

Radiation, conduction and Convection

Insulation

swoabsorbed aAIP sin

Power outPower out

Radiation, Conduction, and Radiation, Conduction, and ConvectionConvection

FactorsFactors• A – Area through which energy flowsA – Area through which energy flows

● ● T – temperature gradient from inside to T – temperature gradient from inside to outsideoutside

• MaterialMaterial U – heat transfer coefficient (radiation, U – heat transfer coefficient (radiation,

conduction, and convection)conduction, and convection)

Power out- detailsPower out- details

PPoutout = UA = UATT sb = Sides and sb = Sides and

BottomBottom w – Windoww – Window io – interior ovenio – interior oven ambient – outside ambient – outside

ovenovenSides/Bottom

Window/glazing

ambientiowwsbsbout TTAUAUP

Balancing Energy (out = in)Balancing Energy (out = in)

Power out = Power absorbedPower out = Power absorbed

swoabsorbed aAIP sin

ambientiowwsbsbout TTAUAUP

Rearranging for Rearranging for TTioio

wwsbsb

swoambientio AUAU

aAITT

sin

Predict Final Oven Temp TPredict Final Oven Temp T ioio

Use an Excel Spreadsheet II0 0 – fixed– fixed Angle of sun – fixedAngle of sun – fixed Position the ovenPosition the oven Window areaWindow area

• Bigger window but Bigger window but heat loss increaseheat loss increase

InsulationInsulation ReflectorsReflectors

Uw = f(Tio)

Tio = f(Uw)

ReflectorsReflectors

Goal is to capture more light and Goal is to capture more light and allow less heat to escapeallow less heat to escape

Solar Oven with ReflectorsSolar Oven with Reflectors

ReflectorsReflectors

Energy GainEnergy Gain• Some solar energy reflected is adsorbed by reflector Some solar energy reflected is adsorbed by reflector

and more heat (energy) retained in ovenand more heat (energy) retained in oven

• PPabsorbed with a reflectorabsorbed with a reflector = G P = G Pabsorbed without a reflectorabsorbed without a reflector

r – reflectivity of reflector

M – height of reflector

– angle of reflected light

N - # of reflectors

90sin1L

MrNG

wwsbsb

woambientio AUAU

aGAITT

M/L RatioM/L Ratio Cannot merely make a Wide/Squat Cannot merely make a Wide/Squat

vs. Tall skinny Pyramidvs. Tall skinny Pyramid• Much of the sun’s rays would miss Much of the sun’s rays would miss

windowwindow